We (VBAM Games) sell the Victory by Any Means campaign system, which is a set of generic sci-fi campaign rules that you can use stand-alone (with strategic combat) or alongside your tactical rules. It is a complete strategy game, which means that it might include more elements in the strategic layer than you want to deal with, but you can always cut out the content that you don't want to use.
The First Edition is currently discounted down to $7.50 USD USD, and the new Second Edition is out this Friday (July 3) for $15 USD USD.
With the utterly shameless shilling out of the way, another approach you might try is something akin to the old Wing Commander Armada computer game worked, where the map is a number of systems connected together by a series of warp lanes and each system generates a fixed amount of points each turn that you can spend. If I remember right, I think the points banked up at each individual location, and you built one thing at a time at a set rate. There were transports that you could build that you would then use to move these resources around, in case you had a Shipyard in one system and needed to feed it resources from another system.
The Armada rules also gave each system a finite amount of resources that it could harvest before it was exhausted, which forced players to continue expanding instead of sitting and turtling indefinitely.
The victory condition in Armada was to destroy the enemy's carrier, which effectively made it a "capture the flag" style scenario.
For a simple tactical campaign, the Armada system isn't a bad one to emulate, and it was fun as a thin strategic layer to the Wing Commander space combat rules.